This invention relates generally to vertical cavity surface-emitting lasers (VCSELs) and, more particularly, to methods of testing bottom-emitting VCSELs.
A VCSEL is a semiconductor laser in which a first multiplicity of semiconductor layers (e.g., Group III-V compound layers) forms an active region (e.g., an MQW active region), which is sandwiched between a second and third multiplicity of layers, which form a pair of mirrors. One mirror, the bottom mirror, is formed under the active region and nearer the substrate, whereas the other mirror, the top mirror, is formed above the active region and farther from the substrate. The mirrors define a cavity resonator having its longitudinal axis oriented perpendicular to the plane of the layers. When the active region is forward biased and pumping current is applied thereto in excess of the lasing threshold, the VCSEL generates stimulated, coherent radiation that is emitted along the resonator axis. The wavelength of the radiation is determined by the bandgap of the material used to form the active region. Thus, for operation at relatively short wavelengths in the range of about 800-1000 nm, the layers of the active region typically comprise GaAs/AlGaAs compounds epitaxially grown on an optically absorbing GaAs substrate, whereas for operation at longer wavelengths of about 1100-1600 nm, the layers typically comprise InP/InGaAsP compounds epitaxially grown on an optically transparent InP substrate.
The radiation may emerge through either or both mirrors depending on their reflectivity A VCSEL is termed a bottom-emitting device if the primary, relatively high intensity, emission is through the bottom mirror. This emission will propagate through the substrate if it is optically transparent. In many designs the substrate is removed and hence, even if it had been optically absorbing, does not obstruct the emission through the bottom mirror. On the other hand, the secondary, much lower intensity, emission that leaks through the top mirror is termed the backside emission. Bottom-emitting VCSELs are attractive because they are known to facilitate flip-chip bonding. In contrast, a VCSEL is termed a top-emitting device if the primary emission is through the top mirror.
One important feature of VCSELs is their ability to be fabricated in an array containing, for example, thousands of lasers. These arrays can be used to provide a multiplicity of carrier sources in fiber optic communication systems; e.g., dense optical interconnect solutions for high-end routers, cross-connects and switching systems. Before an array can be employed in a communications application, or any other application for that matter, it must be tested in order to determine whether each VCSEL, as well as the overall array, satisfies predetermined performance specifications. Defective VCSELs (i.e., those that do not meet specification) result in lower efficiency and wasted power consumption. Optimally an array is tested at a time in the manufacturing process (e.g., before substrate removal or final assembly) that minimizes economic loss should the array fail to meet specification and have to be discarded. To this end in the prior art, a top-emitting VCSEL array is probed in step-and-repeat fashion, one VCSEL at a timexe2x80x94a very time consuming, expensive process. The probe includes driver circuitry for supplying the necessary bias voltage and pumping current to the VCSEL under test and photodetection circuitry for measuring the intensity of the primary emission. Testing bottom-emitting VCSEL arrays is more problematic. For short wavelength bottom-emitting devices, the presence of the absorbing substrate prevents making optical measurements of the primary emission. To our knowledge, therefore, manufacturers limit their testing of short wavelength, bottom-emitting VCSELs to making electrical measurements to identify shorts or open circuits in each VCSEL, again using a step-and-repeat approach. In contrast, the substrate of long wavelength bottom-emitting VCSELs is transparent, but these devices are not currently in commercial manufacture to our knowledge.
Thus, a need remains in the art for an effective technique for testing bottom-emitting VCSELs regardless of their wavelength of operation.
In accordance with one aspect of our invention, an array of bottom-emitting VCSELs, with its substrate still intact, is tested by means of a probe that includes an optoelectronic array, which is aligned and coupled to the top surface of the VCSEL array. The probe is aligned to the VCSEL array just once. The optoelectronic array includes driver circuits for energizing the VCSELs and the photodetection circuits in a predetermined sequence for detecting the back emission that leaks through the top mirror of each VCSEL. In another embodiment, this probe and method are applied to testing bottom-emitting VCSELs one at a time. The VCSELs may be discrete devices or part of an array.
In accordance with another aspect of our invention, an array of bottom-emitting VCSELs, with its substrate still in intact, is tested by means of a probe that includes separate electronic and photodetection arrays. The probe is aligned to the VCSEL array just once. The electronic array, which is electrically coupled to the top surface of the VCSEL array, includes driver circuits for energizing the VCSELs. The photodetection array is aligned and coupled to the bottom of the substrate in order to detect the primary bottom emission of the energized VCSELs. The photodetection array is aligned so that each detector receives the emission from a particular VCSEL, but because the substrate is relatively thick, the divergence of the bottom emission produces cross-talk; that is, the bottom emission of one VCSEL may be received by an adjacent photodetector that is supposed to detect only the emission from another VCSEL. To alleviate this cross-talk problem, the VCSELs are energized in a first predetermined sequence and/or the photodetector circuitry is turned on in a second predetermined sequence; e.g., so that all VCSELs are on concurrently and adjacent photodetectors are not on at the same time. Alternatively, all of the photodetection circuitry may be turned on concurrently, and the VCSELs may be energized in a predetermined sequence to reduce cross-talk. In another example, first groups (e.g., pairs) of VCSELs may be energized in a first predetermined sequence and second groups (e.g., pairs) of photodetection circuits may be energized in a second predetermined sequence so as to reduce cross-talk, with VCSELs in each first group being energized concurrently with one another and circuits in each second group being energized concurrently with one another.
Both aspects of our invention enable testing of an entire array essentially simultaneously, thereby reducing costs of testing to the point that it is feasible to test all VCSEL arrays prior to final assembly. Since only VCSEL arrays that meet specification are assembled, final device yields are improved. Furthermore, and in accordance with another embodiment of our invention, the drive circuits to the VCSELs that do not meet specification are turned off in the final device, thereby reducing power consumption wasted on such VCSELs.